This invention relates generally to DC motors, and, more specifically, to a method and apparatus for controlling a speed of a DC motor.
Modern appliances, such as a refrigerator, typically include a number of devices controlled by DC motors, such as, for example, an evaporator fan or circulation fans within refrigerator compartments. To meet increasingly stringent energy requirements and standards, the motors are to be operated at specific desired speeds. When run without feedback control, DC motor speed undesirably varies with an ambient operating temperature, leading to some performance fluctuation between substantially identical appliances with substantially identical motors. Using feedback control, uniform performance across a product line may be achieved and energy requirements may be satisfied despite fluctuating operating conditions.
Known feedback speed control methods for DC motors, however, tend to be relatively complicated and involve a variety of computations to be performed in small time periods. See, for example, U.S. Pat. No. 4,371,819. Thus, known feedback control methods present an appreciable computational load on a motor controller, which increases control complexity and decreases control response time to changing motor conditions.
Accordingly, it would be desirable to provide a feedback control method and apparatus for a DC motor that reduces a computational load on the controller, simplifies the control scheme, and increases control response time.
In an exemplary embodiment, a method for maintaining a target speed of a DC motor having a rotatable motor shaft driven by a controller supplying a pulse-width variable drive signal includes the steps of obtaining a pulse feedback signal from the motor that corresponds to a number of revolutions of the motor shaft, measuring an actual time for a predetermined number of feedback pulses to be received by the controller, comparing the actual time for the predetermined number of pulses to be received with an expected time for the feedback pulses to be received when the rotor shaft is operating at the target speed, and adjusting the controller pulse-width variable signal in response to the compared actual time to the expected time. The width of the pulse-width variable signal is increased when the actual time is greater than the expected time, and decreased when the actual time is less than the expected time.
More specifically, a feedback signal including four pulses per revolution of the motor shaft is obtained, and an actual time to receive twenty feedback pulses is measured. If the actual time to receive a number of feedback pulses to be received exceeds a predetermined time period, a stalled motor is indicated and the pulse-width drive signal is increased to restart the motor. A proportional-integral control scheme based upon a difference, or error, between the expected time and the measured actual time is used to adjust the pulse-width drive signal when twenty pulses are counted within the predetermined time period. The drive signal is generated and adjusted by a microprocessor in response to the feedback pulses generated by a feedback element coupled to the motor.
Measuring an elapsed time for twenty feedback pulses to be received has an averaging effect on motor speed measurement, reduces a computational frequency, simplifies the control scheme, and accordingly reduces a computational load on the microprocessor relative to known speed control methods.